The invention relates generally to gas turbine engines, and, more specifically, to film cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT), which powers the compressor, and in a low pressure turbine (LPT), which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof to achieve a long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. In addition, all of these components typically include common rows of film cooling holes.
A typical film cooling hole is a cylindrical bore inclined at a shallow angle through the heated wall for discharging a film of cooling air along the external surface of the wall to provide thermal insulation against the hot combustion gases which flow thereover during operation. The film is discharged at a shallow angle over the wall outer surface to minimize the likelihood of undesirable blow-off thereof, which would lead to flow separation and a loss of the film cooling effectiveness. Film cooling holes are typically arranged in rows of closely spaced apart holes, which collectively provide a large area cooling air blanket over the external surface. However, the more holes required to provide full-surface coverage of the film cooling boundary layer, the more air is also required, thereby decreasing engine efficiency.
At present, film cooling holes formed in hot gas path components utilize straight holes and straight facet features. For example, diffuser shaped holes 2 are made with straight round holes 4 and a straight shaped exit footprint 6 at a differing angle. FIGS. 1-4 schematically illustrate prior art diffuser film cooling holes. FIGS. 1 and 2 depict a prior art laid back fan diffuser film cooling hole, where D is the diameter of the straight round hole 4, LT is the length of the straight round hole 4, L is the entire length of the diffuser film cooling hole, δ is the angle between an inboard surface 5 of the exit portion 6 of the diffuser and a centerline 7, and α is the angle between the centerline 7 and a straight outer surface 8 of the film cooled wall 3. Referring to FIG. 2, β is the angle between the centerline 7 and the inboard surface 5 of the exit portion 6 of the diffuser film cooling hole. As can be seen in FIGS. 1 and 2, the prior art laid back fan diffuser film cooling hole uses straight surface facets in the direction of the film hole and flow. Similarly, FIGS. 3 and 4 schematically depict a prior art fan diffuser film cooling hole (also indicated by reference numeral 2), and the same reference numerals are used to indicate the corresponding features in FIGS. 1-4. As can be seen in FIGS. 3 and 4, the prior art fan diffuser film cooling hole also uses straight surface facets in the direction of the film hole and flow. The conventional fan diffuser cooling holes shown in FIGS. 1-4 are typically formed by electric discharge machining (EDM) with conventional electrodes.
These straight surface facets lead to high injection angles and significant film blow-off for conventional film cooling holes with exit portions on curved surfaces of the hot gas path component. It would therefore be desirable to provide film cooling holes with reduced film-blow-off for use in hot gas path components with film cooled curved surfaces.